1. Field of Invention
This invention relates to electrical switches and more particularly it relates to an electrical apparatus which permits a normally open electrical switch to perform the function of a normally closed switch. It has particular application to the use of low voltage switches of the type referred to generally as membrane switches, which are all normally open, to perform the function of a normally closed switch in low voltage electronic circuits.
2. Background Information
A type of electrical switch which is widely used today in low voltage applications is the so called "membrane" switch. This is a general classification of mechanically simple electrical switches used with low voltage, such as 5 volt, electronic circuits. Such circuits are widely used today in logic and control circuits where the active elements are solid state devices such as transistors which operate at these low voltages.
Because of the low voltages at which the membrane switches operate, they do not require a great deal of insulation, and do not need much opening separation since the voltage and current drawn by the circuits controlled by them are not high enough for arcing to become a problem. In their simplest form, they can be two parallel conductors biased apart by a resilient member. Pressure applied to one conductor brings the two into contact to close the switch. In another form, one contact is an electrically conductive concave disc which is positioned over a second contact. Often the switch elements are covered by an insulating membrane which also environmentally isolates the switch. Such switches are widely used such as in keyboards for electronic components and in many other applications.
A drawback of these simple low voltage switches is that they are only available as normally open switches. They are push button switches which only remain closed as long as pressure is applied. There are some applications, however, where it would be desirable to use these simple low voltage so called membrane switches, but a normally closed switch is required. One such application is a contactor or starter for motors. Typically, these devices include a stop switch which is normally closed. Such a switch which is sometimes referred to as a run permit switch provides power to a start switch which is a normally open switch. With the stop switch closed, actuation of the start switch provides power to the controlled device. The start switch normally seals in an energizing circuit so that the switch can be released. When it is desired to deenergize the controlled device, the normally closed stop or run permit switch is pressed to open the energizing circuit and break the seal in so that when the switch is released, the controlled device remains deenergized.
There exists a need therefore for a normally open switch device which utilizes low cost, low voltage normally open membrane type switches.
There is a further need for such a device which is itself simple and economical and is reliable.